Chiral smectic liquid crystal device having fluorine-containing polymeric alignment film with predetermined refractive index anisotropy after rubbing

ABSTRACT

A liquid crystal device is constituted by a pair of substrates, and a liquid crystal disposed between the substrates. At least one of the substrates has thereon an alignment film. The alignment film is characterized in that (1) it has a surface energy of at most 35 dyne/cm before rubbing and has been rubbed to have a refractive index anisotropy of at least 0.02, or (2) it has been rubbed to have a surface energy difference ΔE of at least 9 dyne/cm and a difference Δδ in dispersion term of the surface energy of at least 6 dyne/cm between before and after the rubbing. The alignment film is effective in providing a high contrast and decreasing a delay in optical response causing after-image.

This application is a continuation of application Ser. No. 08/272,846,filed Jul. 11, 1994, now abandoned, which is a division of applicationSer. No. 08/180,459, filed Jan. 12, 1994, now U.S. Pat. No. 5,400,159,which is a division of application Ser. No. 07/925,200, filed Aug. 6,1992, now abandoned.

FIELD OF THE INVENTION AND RELATED ART

This invention relates to a liquid crystal device to be used in a liquidcrystal display device or a liquid crystal-optical shutter, etc.,particularly a liquid crystal device by use of a ferroelectric liquidcrystal, more particularly to a liquid crystal device improved indisplay characteristics through improvement in initial alignment of theliquid crystal molecules.

A display device of the type which controls transmission of light incombination with a polarizing device by utilizing the refractive indexanisotropy of ferroelectric liquid crystal molecules has been proposedby Clark and Lagerwall (U.S. Pat. No. 4,367,924, etc.). Theferroelectric liquid crystal has generally chiral smectic C phase (SmC*)of H phase (SmH*) of a non-helical structure and, under this state,shows a property of taking either one of a first optically stable stateand a second optically stable state responding to an electrical fieldapplied thereto and maintaining such a state in the absence of anelectrical field, namely bistability, and also has a rapid response tothe change in electrical field. Thus, it is expected to be utilized in ahigh speed and memory type display device and particularly to provide alarge-area, high-resolution display.

For an optical modulating device by use of a liquid crystal having suchbistability to exhibit desirable driving characteristics, it is requiredthat the liquid crystal disposed between a pair of substrates should bein such a molecular alignment state that conversion between the abovetwo stable states may occur effectively irrespective of the applicationof an electrical field.

Further, in a liquid crystal device utilizing birefringence of a liquidcrystal, the transmittance under right angle cross nicols is given bythe following equation:

    I/I.sub.0 =sin.sup.2 4θ sin.sup.2 (Δnd/λ)π,

wherein

I₀ : incident light intensity,

I: transmitted light intensity,

θ: tilt angle,

Δn: refractive index anisotropy,

d: thickness of the liquid crystal layer,

λ: wavelength of the incident light.

The tilt angle θ in the above-mentioned non-helical structure isrecognized as a half of an angle between the average molecular axisdirections of liquid crystal molecules in a twisted alignment in a firstorientation state and a second orientation state. According to the aboveequation, it is shown that a tilt angle θ of 22.5 degrees provides amaximum transmittance and the tilt angle in a non-helical structure forrealizing bistability should desirably be as close as possible to 22.5degrees.

A method for aligning a ferroelectric liquid crystal should desirably besuch that molecular layers each composed of a plurality of molecules ofa smectic liquid crystal are aligned uniaxially along their normals, andit is desirable to accomplish such an alignment state by a rubbingtreatment which requires only a simple production step.

As an alignment method for a ferroelectric liquid crystal, particularlya chiral smectic liquid crystal in a non-helical structure, onedisclosed in U.S. Pat. No. 4,561,726 has been known for example.

However, when a conventional alignment method, particularly one using apolyimide film or polyamide film treated by rubbing, is applied foralignment of a ferroelectric liquid crystal in a non-helical structureexhibiting bistability reported by Clark and Lagerwall, the followingproblems are encountered.

That is, according to our experiments, it has been found that a tileangle θ (an angle shown in FIG. 3 as described below) in a ferroelectricliquid crystal with a non-helical structure obtained by alignment withan alignment control film of the prior art has become smaller ascompared with a tilt angle H (an angle shown in FIG. 2 as describedbelow) in the ferroelectric liquid crystal having a helical structure.Particularly, the tilt angle θ in a ferroelectric liquid crystal with anon-helical structure obtained by alignment with alignment control filmsof the prior art was found to be generally on the order of 3-8 degrees,and the transmittance at that time was at most about 3 to 5%.

Thus, according to Clark and Lagerwall, the tilt angle in aferroelectric liquid crystal with a non-helical structure realizingbistability should have the same angle as the tilt angle in theferroelectric liquid crystal having a helical structure, but in fact thetilt angle θ in a non-helical structure is smaller than the tilt angle Hin a helical structure. More specifically, it has been found that thetilt angle θ in a non-helical structure becomes smaller than the tiltangle H because of a twist alignment of liquid crystal molecules in thenon-helical structure. Thus, in a ferroelectric liquid crystal having anon-helical structure, liquid crystal molecules are aligned with a twistfrom a molecular axis adjacent to an upper substrate to a molecular axisadjacent to a lower substrate continuously at a certain twist angle.This leads to a phenomenon that the tilt angle θ in the non-helicalstructure is smaller than the tilt angle H in the helical structure.

Further, in an alignment state of a chiral smectic liquid crystalattained by a conventional polyimide or polyamide alignment filmsubjected to a rubbing treatment, when a liquid crystal is supplied witha voltage of one polarity for switching from a first optically stablestate (e.g., a white display state) to a second optically stable state(e.g., a black display state) and then the voltage of one polarity isremoved, the ferroelectric liquid crystal layer is supplied with areverse electric field Vrev due to the presence of the polyimide film asan insulating layer between the electrode and the liquid crystal layer,and the reverse electric field Vrev has caused an after-image duringdisplay. The generation of the above-mentioned reverse electric fieldhas been reported in "Switching characteristic of SSFLC" by AkioYoshida, "Preprint for Liquid Crystal Forum, October 1987" p.p. 142-143.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide aferroelectric liquid crystal device having solved the above-mentionedproblems, particularly a ferroelectric liquid crystal device whichprovides a large tilt angle θ of a chiral smectic liquid crystal in anon-helical structure and provides a display capable of displaying ahigh-contrast image and yet free from after-image.

According to the present invention, there is provided a liquid crystaldevice, comprising a pair of substrates, at least one of which has analignment film thereon, and a liquid crystal disposed between thesubstrates; wherein said alignment film has a surface energy of at most35 dyne/cm before rubbing and has been rubbed to have a refractive indexanisotropy of at least 0.02.

According to another aspect of the present invention, there is provideda liquid crystal device, comprising a pair of substrates, at least oneof which has an alignment film thereon, and a liquid crystal disposedbetween the substrates; wherein said alignment film has a surface energydifference ΔE of at least 9 dyne/cm and a difference Δδ in dispersionterm of the surface energy of at least 6 dyne/cm between before andafter the rubbing.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an embodiment of the liquidcrystal device according to the present invention.

FIG. 2 is a perspective view showing schematically an alignment of achiral smectic liquid crystal having a helical structure. FIG. 3 is aperspective view showing schematically an alignment state of a chiralsmectic liquid crystal having a non-helical structure.

FIG. 4 is a schematic sectional view showing an alignment state of achiral smectic liquid crystal aligned according to the presentinvention.

FIG. 5 is an illustration of C-director alignments in a uniformalignment state.

FIG. 6 is an illustration of C-director alignments in a splay alignmentstate.

FIGS. 7A and 7B are plan views illustrating tilt angles θ in a uniformalignment state and a splay alignment state, respectively.

FIG. 8 is a sectional view showing a charge distribution, a direction ofa spontaneous polarization P_(S) and a direction of a reverse electricfield Vrev.

FIG. 9 is a schematic plan view illustrating changes in tilt angle θduring and after application of an electric field.

FIGS. 10 and 11 are graphs showing optical response characteristicsaccording to a conventional device and the present invention,respectively.

FIG. 12 is a waveform diagram illustrating driving waveforms used in anembodiment of the present invention.

FIGS. 13 and 14 are respectively an illustration of rubbing directionsintersecting with each other.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic sectional view of an embodiment of the liquidcrystal device according to the present invention.

The liquid crystal device comprises a pair of substrates (glass plates)11a and 11b which are coated with transparent electrodes 12a and 12b ofIn₂ O₃, ITO (indium tin oxide), etc., 200-1500 Å-thick insulating films13a of SiO₂, TiO₂, Ta₂ O₅, etc., and 50-1000 Å-thick alignment controlfilms 14a and 14b of a polyimide or polyamide.

In this instance, the alignment control films 14a and 14b have beentreated by rubbing in directions which are parallel to each other and inthe same direction (indicated by arrows A in FIG. 1A). A chiral smecticliquid crystal 15 is disposed between the substrates 11a and 11b, andthe spacing between the substrates 11a and 11b is set to provide theferroelectric liquid crystal layer 15 with a thickness (e.g., 0.1-3microns) which is sufficiently small to suppress the formation of ahelical structure of the chiral smectic liquid crystal 15 by disposingspacer beads 16 of, e.g., silica, alumina, etc. between the substrates11a and 11b, whereby the chiral smectic liquid crystal 15 assumes abistable alignment state. The cell structure thus formed is sandwichedbetween a pair of polarizers 17a and 17b.

In the present invention, the rubbing may preferably be applied to bothof the alignment films 14a and 14b. In this instance, the rubbingdirections to the alignment films 14a and 14b may be parallel identicaldirections as described above, parallel reverse directions, or slightlycrossing identical directions forming an intersection angle of 2-20degrees.

In the present invention, it has been found suitable to use a polyimideor polyamide alignment film having a low surface energy of at most 35dyne/cm, preferably at most 32 dyne/cm and a refractive index anisotropy(Δn) which increases to a certain extent by rubbing so as to provide aferroelectric liquid crystal device showing a large tilt angle and gooddisplay characteristics. As for the increase in refractive indexanisotropy, an alignment film showing a smaller increase on the samesurface energy level tends to provide better display characteristics.

Herein, the surface energy refers to a value measured by the sessiledrop method using a contact angle meter, and the evaluation of therefractive index anisotropy is based on measurement by means of ahigh-sensitivity automatic birefringence meter using a photoelasticitymodulation element.

The alignment film used in the present invention is not required to becomposed of a material having a specific chemical structure as far as itsatisfies the above-mentioned conditions, but may suitably be composedof materials enumerated for example below.

Thus, examples of tetracarboxylic acid components constitutingpolyimides may include pyromellitic dianhydride, biphenyltetracarboxylicdianhydride, naphthalenetetracarboxylic dianhydride, andcyclobutanetetracarboxylic dianhydride.

Examples of dicarboxylic acid components constituting polyamides mayinclude: terephthalic acid, 4,4'-biphenyldicarboxylic acid, and2,6-naphthalenedicarboxylic acid.

Further, examples of diamines constituting the polyimide or polyamidemay include bis[4-(aminophenoxy)phenyl] compounds represented by thefollowing formula (I): ##STR1## wherein R₁ and R₂ independently denotefluoroalkyl chains, such as CF₃ (CF₂)_(m) (CH₂)_(L) - (wherein L≧0,m≧0), and R₁ and R₂ may be the same or different.

In order to form a film of the polyimide or polyamide on a substrate, asolution of a polyamide acid as a precursor of the polyimide or thepolyamide per se in a solvent, such as dimethylformamide,dimethylacetamide, dimethylsulfoxide, or N-methyl-2-pyrrolidone, at aconcentration of 0.01-40 wt. %, may be applied onto the substrate byspinner coating, spray coating, roller coating, etc., and heated to100°-350° C., preferably 200°-300° C., for dehydrocyclization to formthe polyimide film or for evaporation of the solvent to form thepolyamide film. The thus-formed polyimide or polyamide film may berubbed with a cloth, etc. The polyimide or polyamide film used in thepresent invention may be formed in a thickness of about 30 Å-1 μm,preferably 200-2000 Å. In this case, the insulating films 13a and 13bshown in FIG. 1 can be omitted. Further, in the case of forming thepolyimide or polyamide film on the insulating film 13a or 13b, thepolyimide or polyamide film may be formed in a thickness of at most 500Å, preferably at most 300 Å.

The liquid crystal material used in the present invention may preferablybe one showing a phase transition from isotropic phase throughcholesteric phase and smectic A phase into chiral smectic C phase in thecourse of temperature decrease. Particularly, a chiral smectic liquidcrystal showing a helical pitch of 0.8 microns or longer in cholestericphase (measured at a mid temperature in the cholesteric range).Preferred examples of such a liquid crystal material may include liquidcrystal materials (1)-(5) below comprising the following liquid crystals"LC-1", "80B" and "80SI*" in the indicated proportions by weight.##STR2##

Liquid Crystal Material

(1) (LC-1)₉₀ /(80B)₁₀

(2) (LC-1)₈₀ /(80B)₂₀

(3) (LC-1)₇₀ (80B)₃₀

(4) (LC-1)₆₀ /(80B)₄₀

(5) (80SI*)₁₀₀

FIG. 2 is a schematic illustration of a ferroelectric liquid crystalcell (device) for explaining operation thereof. Reference numerals 21aand 21b denote substrates (glass plates) on which a transparentelectrode of, e.g., In₂ O₃, SnO₂, ITO (indium-tin-oxide), etc., isdisposed, respectively. A liquid crystal of an SmC*-phase (chiralsmectic C phase) or SmH*-phase (chiral smectic H phase) in which liquidcrystal molecular layers 22 are aligned perpendicular to surfaces of theglass plates is hermetically disposed therebetween. Full lines 23 showliquid crystal molecules, and disposed on cones forming in succession ahelical structure in the direction of extension of the substrates. Ahalf of the apex angle of the cone provides a tilt angle H in thehelical structure of the chiral smectic phase. Each liquid crystalmolecule 23 has a dipole moment (P.sub.⊥) 24 in a directionperpendicular to the axis thereof. When a voltage higher than a certainthreshold level is applied between electrodes formed on the substrates21a and 21b, a helical structure of the liquid crystal molecule 23 isunwound or released to change the alignment direction of respectiveliquid crystal molecules 23 so that the dipole moment (P₋₋) 24 are alldirected in the direction of the electric field. The liquid crystalmolecules 23 have an elongated shape and show a refractive anisotropybetween the long axis and the short axis thereof. Accordingly, it iseasily understood that when, for instance, polarizers arranged in across nicol relationship, i.e., with their polarizing directionscrossing each other, are disposed on the upper and the lower surfaces ofthe glass plates, the liquid crystal cell thus arranged functions as aliquid crystal optical modulation device of which opticalcharacteristics vary depending upon the polarity of an applied voltage.

Further, when the liquid crystal cell is made sufficiently thin (e.g.,0.1-3 microns), the helical structure of the liquid crystal molecules isunwound to provide a non-helical structure even in the absence of anelectric field, whereby the dipole moment assumes either of the twostates, i.e., Pa in an upper direction 34a or Pb in a lower direction34b as shown in FIG. 3, thus providing a bistable condition. When anelectric field Ea or Eb higher than a certain threshold level anddifferent from each other in polarity as shown in FIG. 3 is applied to acell having the above-mentioned characteristics, the dipole moment isdirected either in the upper direction 34a or in the lower direction 34bdepending on the vector of the electric field Ea or Eb. Incorrespondence with this, the liquid crystal molecules are oriented ineither of a first stable state 33a and a second stable state 33b. A halfof the angle between the first and second stable states corresponds to atilt angle θ.

A first advantage attained by using such a ferroelectric liquid crystalcell is that the response speed is quite fast, and a second advantage isthat the orientation of the liquid crystal shows bistability. The secondadvantage will be further explained, e.g., with reference to FIG. 3.When the electric field Ea is applied to the liquid crystal molecules,they are oriented in the first stable state 33a. This state is stablyretained even if the electric field is removed. On the other hand, whenthe electric field Eb of which direction is opposite to that of theelectric field Ea is applied thereto, the liquid crystal molecules areoriented to the second stable state 33b, whereby the directions ofmolecules are changed. This state is similarly stably retained even ifthe electric field is removed. Further, as long as the magnitude of theelectric field Ea or Eb being applied is not above a certain thresholdvalue, the liquid crystal molecules are placed in the respectiveorientation states.

FIG. 4 is a schematic sectional view showing an alignment state ofliquid crystal molecules attained by the present invention, and FIG. 5is a view showing alignment of corresponding C-directors.

Reference numerals 51a and 51b in FIG. 4 denote upper and lowersubstrates, respectively. Numeral 50 denotes a molecular layer composedof liquid crystal molecules 52, and liquid crystal molecules 52 arealigned so as to change their positions along the bottom face 54(circular) of a cone 54. FIG. 5 more specifically shows a change inC-directors. Referring to FIG. 5, at U₁ are shown C-directors 81 (eachbeing a projection of a molecular long axis onto an imaginary planeperpendicular to the normal to a molecular layer 50) in one stableorientation state, and at U₂ are shown C-directors 81 in the otherstable orientation state.

On the other hand, an alignment state attained by a conventionalrubbing-treated polyimide film may be represented by a C-directordiagram of FIG. 6, which shows an alignment state wherein molecular axesare twisted in a large degree from the upper substrate 51a to the lowersubstrate 51b to provide a smaller tilt angle θ.

FIG. 7A is a schematic plan view illustrating a tilt angle θ in analignment state where C-directors 81 assume a state shown in FIG. 5(referred to as "uniform alignment state"), and FIG. 7B is a schematicplan view illustrating a tilt angle θ in an alignment state whereC-directors 81 assume a state shown in FIG. 6 (referred to as "splayalignment state"). In these figures, reference numeral 60 denotes arubbing axis provided to the above-mentioned specific polyamide orpolyamide film according to the present invention, numeral 61a denotesan average molecular axis in the orientation state U₁, numeral 61bdenotes an average molecular axis in the orientation state U₂, numeral62a denotes an average molecular axis in the orientation state S₁, andnumeral 62b denotes an average molecular axis in the orientation stateS₂. The average molecular axes 61a and 61b can be switched to each otherby applying voltages of mutually opposite polarities. Similar switchingis caused between the average molecular axes 62a and 62b.

Next, the effectiveness of the uniform alignment state with respect to adelay in optical response (after-image) due to a reverse electric fieldVrev is explained.

If the capacitance of an insulating layer constituting a liquid crystalcell is denoted by Ci, the capacitance of a liquid crystal layer isdenoted by C_(LC) and the spontaneous polarization of the liquid crystalis denoted by P_(S), Vrev causing after-image is expressed by thefollowing equation.

    Vrev=2P.sub.S /(Ci+C.sub.LC)

FIG. 8 is a schematic sectional view illustrating changes in chargedistribution direction of P_(S) and direction of the reverse electricfield in a liquid crystal cell. At FIG. 8(a), there is shown adistribution of ⊕ and ⊖ charges in a memory state before application ofa pulse electric field, where the spontaneous polarization is directedfrom ⊕ charges to ⊖ charges. At FIG. 8(b) is shown a state immediatelyafter removal of a pulse electric field, when the direction of thespontaneous polarization P_(S) is opposite to that shown at FIG. 8(a)(thus, the liquid crystal molecules are inverted from one stableorientation state to the other orientation state) but the distributionof the ⊕ and ⊖ charges is similar to that shown at FIG. 8(a), so that areverse electric field Vrev is generated as indicated by an arrow shownat FIG. 8(b). The reverse electric field Vrev disappears in a short timeto provide a distribution of ⊕ and ⊖ charges as shown at FIG. 8(c).

FIG. 9 is a plan view showing a change in optical response in a splayalignment state given by a conventional polyimide or polyamide alignmentfilm in terms of a change in tilt angle θ. Referring to FIG. 9, at thetime of application of a pulse electric field, the orientation of liquidcrystal molecules is changed from an average molecular axis S(A) in asplay alignment state to be overshot to an average molecular axis U₂ ina uniform alignment state close to that providing a maximum tilt angle Halong a path denoted by an arrow X₁, and immediately after the removalof the pulse electric field, the orientation is changed along a pathdenoted by an arrow X₂ to an average molecular axis S(B) in a splayalignment state providing a decreased tilt angle θ due to the action ofthe reverse electric field Vrev shown at FIG. 8(b). Then, as the reverseelectric field Vrev attenuates as shown at FIG. 8(c), the orientation ischanged along a path denoted by an arrow X₃ to an average molecular axisS(C) in a splay alignment state providing a stable orientation statehaving a somewhat increased tilt angle θ. FIG. 10 is a graph showing anoptical response at this time.

In the present invention using an alignment film having a low surfaceenergy of at most 35 dyne/cm before rubbing or in an as-coated state andcaused to have an increased refractive index anisotropy (Δn) by rubbingas described above, the average molecular axes S(A), S(B) and S(C) inthe splay alignment state shown in FIG. 9 are not caused but it ispossible to form an alignment state with an average molecular axisgiving a tilt angle θ which is close to a maximum tilt angle H. Anoptical response at this time is shown in FIG. 11. FIG. 11 shows that adelay in optical response causing after-image is obviated, and a highcontrast in memory states is caused.

Thus, in the liquid crystal device according to the present inventionusing a rubbing-treated specific polyimide or polyamide film as aligningmeans, it has become possible to realize an alignment state whichprovides a large optical contrast between the bright and dark states,particularly a large contrast in non-selected pixels during multiplexingdrive as taught by U.S. Pat. No. 4,655,561, and is free from delay inoptical response at the time of switching (multiplex driving) causingafter-image during display.

Hereinbelow, the present invention will be explained based on Examples.

EXAMPLE 1

Two 1.1 mm-thick glass plates each provided with a 1000 Å-thick ITO filmwere respectively coated with a 3.0 wt. % solution of a polyamide acidrepresented by the formula (II) shown below in a mixture solvent ofN-methylpyrrolidone/n-butylcellosolve=5/1 by spin coating. ##STR3##After the coating, the film was subjected to curing under heating at250° C. for about an hour to form a 226 Å-thick film.

The coating film thus formed was subjected to measurement of a contactangle by the sessile drop method using a contact angle meter (availablefrom Kyowa Kaimen Kagaku K.K.), from which the surface energy Δ_(s)(=Δ^(d) +Δ^(p) +Δ^(h), wherein Δ^(d) : dispersion term, Δ^(p) : polarityterm, and Δ^(h) : hydrogen bond term) was calculated to be 31.4 dyne/cm.

Further, the coating film was found to have a refractive indexanisotropy Δn=0.0066 as measured by using a high-sensitivity automaticbirefringence meter (available from OPC Seisakusho K.K.).

The coating film was then rubbed in one direction with a nylon-plantedcloth. The rubbed coating film showed a refractive index anisotropyΔn=0.0221, and thus the change in Δn due to the rubbing was 0.0155.

On one of the two substrates thus treated, alumina beads with an averagediameter of 1.5 μm were dispersed, and the other substrate wassuperposed thereon so that their rubbing axes were parallel to eachother and disposed in the same direction to form a blank cell.

The blank cell was filled with a ferroelectric smectic liquid crystal("CS-1014" (trade name), available from Chisso K.K.) under vacuum and,after sealing, was gradually cooled from isotropic phase at a rate of 5°C./hour to 30° C., whereby an alignment was effected. The "CS-1014"liquid crystal in the cell showed the following phase transition series.##STR4## Iso.: isotropic phase, Ch.: cholesteric phase,

SmA: smectic A phase,

SmC*: choral smectic C phase.

The above-prepared liquid crystal cell was sandwiched between a pair of90 degrees-cross nicol polarizers to provide a liquid crystal device andwas supplied with a pulse of 50 μsec and 30 V. Then, the cross nicolpolarizers were set at the extinction position (providing the darkeststate), and the transmittance through the liquid crystal device at thistime was measured by a photo-multiplier. Then, a pulse of 50 μsec and-30 V was applied to the device, and the transmittance (brightest state)at this time was measured in the same manner, whereby the following datawere obtained.

Tilt angle θ=15 degrees, transmittance in the brightest state=28%,transmittance in the darkest state=1%, contrast ratio=28:1.

The delay in optical response causing after-image was 0.2 sec or less.

The liquid crystal device was subjected to multiplexing drive fordisplay using driving waveforms shown in FIG. 12, whereby a high-qualitydisplay with a high contrast was attained. Further, after an imagedisplay of a prescribed character image, the whole picture area waserased into "white", whereby no after-image was recognized. Referring toFIG. 12, at S_(N), S_(N+1) and S_(N+2) are shown voltage waveformsapplied to scanning lines, at I is shown a voltage waveform applied to arepresentative date line, and at (I-S_(N)) is shown a combined voltagewaveform applied to the data line I and the scanning line S_(N). In theabove embodiment, the drive was performed under the conditions of V₀=5-8 volts and ΔT=20-70 μsec.

EXAMPLE 2

A liquid crystal device was prepared and evaluated in the same manner asin Example 1 except that polyimide alignment films prepared from apolyamide acid represented by the following structural formula (III)were used: ##STR5##

The alignment film before rubbing showed a surface energy Δ_(s) =31.1dyne/cm and a refractive index anisotropy Δn=0.0154 and, after rubbing,showed a refractive index anisotropy Δn=0.0241, thus showing a change inΔn of 0.0087 due to the rubbing.

The resultant device showed a contrast ratio of 35:1 and a delay time inoptical response of 0.1 sec.

As a result of the multiplexing drive in the same manner as in Example1, the liquid crystal device showed similarly good results as in Example1 with respect to contrast and after-image.

EXAMPLE 3

A liquid crystal device was prepared and evaluated in the same manner asin Example 1 except that polyimide alignment films prepared from apolyamide acid represented by the following structural formula (IV) wereused: ##STR6##

The alignment film before rubbing showed a surface energy Δ_(s) =30.5dyne/cm and a refractive index anisotropy Δn=0.0122 and, after rubbing,showed a refractive index anisotropy Δn=0.0454, thus showing a change inΔn of 0.0332 due to the rubbing.

The resultant device showed a contrast ratio of 32:1 and a delay time inoptical response of 0.1 sec.

As a result of the multiplexing drive in the same manner as in Example1, the liquid crystal device showed similarly good results as in Example1 with respect to contrast and after-image.

EXAMPLE 4

A liquid crystal device was prepared and evaluated in the same manner asin Example 1 except that alignment films of a polyamide represented bythe following structural formula (V) were used: ##STR7##

The alignment film before rubbing showed a surface energy Δ_(s) =33.4dyne/cm and a refractive index anisotropy Δn=0.0333 and, after rubbing,showed a refractive index anisotropy Δn=0.0600, thus showing a change inΔn of 0.0267 due to the rubbing.

The resultant device showed a contrast ratio of 20:1 and a delay time inoptical response of 0.5 sec.

As a result of the multiplexing drive in the same manner as in Example1, the liquid crystal device showed similarly good results as in Example1 with respect to contrast and after-image.

COMPARATIVE EXAMPLES 1-3

Liquid crystal devices were prepared and evaluated in the same manner asin Example 1 except that each pair of alignment films were composed of apolyamide (Comparative Example 1) or polyimides (Comparative Examples 2and 3) respectively represented by the formulae shown below: ##STR8##

The respective alignment films showed a surface energy Δ_(s) and arefractive index anisotropy Δn before rubbing, and a refractive indexanisotropy Δn after rubbing and a change in Δn due to rubbing shown inTable 1 below. The performances of the resultant liquid crystal devicesregarding the contrast ratio and delay in optical response are alsoshown in Table 1 below.

                  TABLE 1                                                         ______________________________________                                                                Device                                                Alignment film          performance                                           Before rubbing After rubbing         Delay                                    .sup.γ s (dyne/      Δn                                                                             Contrast                                                                             time                                 cm)        Δn                                                                              Δn                                                                              change ratio  (sec)                                ______________________________________                                        Comp. 41.0     0.0542  0.0188                                                                              -0.0354                                                                              7:1    2.0                                Ex. 1                                                                         Comp. 36.7     0.0124  0.0156                                                                              0.0032 10:1   1.2                                Ex. 2                                                                         Comp. 42.7     0.0207  0.0249                                                                              0.0042 8:1    2.2                                Ex. 3                                                                         ______________________________________                                    

As is understood from the results of the above Examples and ComparativeExamples, remarkable improvements in contrast ratio and delay time ofoptical response, particularly regarding contrast during multiplexingdrive, were realized by using an alignment film showing a surface energyof at most 35 dyne/cm, preferably at most 32 dyne/cm, and a refractiveindex anisotropy Δn after rubbing of at least 0.02, preferably in therange of 0.025-0.075.

EXAMPLE 5

Two 1.1 mm-thick glass plates each provided with 1000 Å-thick ITOelectrodes were respectively coated with a 2.5 wt. % solution of apolyimide precursor ("LQ1802" available from Hitachi Kasei K.K.) in amixture solvent of N-methylpyprrolidone/n-butylcellosolve=1/1 by meansof a spinner rotating at 3000 rpm, followed by heating at 250° C. for 1hour to form a 400 Å-thick coating film. The coating film was thenrubbed in one direction to show a surface energy of 43.9 dyne/cm whichwas larger by 11.6 dyne/cm than the value before the rubbing. Theincrease in dispersion term due to the rubbing was 11.2 dyne/cm.

On one of the two substrates thus prepared, alumina beads with anaverage diameter of about 1.5 μm were dispersed, and the other substratewas superposed thereon so that their rubbing directions extended inalmost the same directions but the rubbing direction of the uppersubstrate deviated by 6 degrees anti-chlockwise from that of the lowersubstrate as shown in FIG. 13 to form a blank cell.

The blank cell was then filled with a phenyl-pyrimidine-based mixtureliquid crystal under vacuum and, after sealing, was gradually cooledfrom isotropic phase at a rate of 1° C./hour to 30° C., whereby analignment was effected. More specifically, the phenyl-pyrimidine-basedmixture liquid crystal was one showing a tilt angle of 15.4 degrees atroom temperature, a spontaneous polarization of -6.8 nC/cm², a tiltangle θ of 15.4 degrees and a pitch of -17 μm in chiral smectic C phase,and the following phase transition series: ##STR9##

The above-prepared liquid crystal cell was sandwiched between a pair of90 degrees-cross nicol polarizers to provide a liquid crystal device andwas supplied with a pulse of 50 μsec and 30 V. Then, the cross nicolpolarizers were set at the extinction position (providing the darkeststate), and the transmittance through the liquid crystal device at thistime was measured by a photo-multiplier. Then, a pulse of 50 μsec and-30 V was applied to the device, and the transmittance (brightest state)at this time was measured in the same manner, whereby the following datawere obtained.

Tilt angle θ=12 degrees, transmittance in the brightest state=36%,transmittance in the darkest state=1.2%, contrast ratio=30:1.

The liquid crystal device was subjected to multiplexing drive fordisplay using driving waveforms shown in FIG. 12, whereby a high-qualitydisplay with a high contrast was attained. Referring to FIG. 12, atS_(N), S_(N+1) and S_(N+2) are shown voltage waveforms applied toscanning lines, at I is shown a voltage waveform applied to arepresentative date line, and at (I-S_(N)) is shown a combined voltagewaveform applied to the data line I and the scanning line S_(N). In theabove embodiment, the drive was performed under the conditions of V₀=5-8 volts and ΔT=20-70 μsec.

EXAMPLE 6

A liquid crystal device was prepared and evaluated in the same manner asin Example 5 except that alignment films were prepared from anotherpolyimide precursor ("LX-S401" (trade name), available from HitachiKasei K.K.). The contrast and surface energy values obtained thereby areshown in Table 2 below.

As a result of the multiplexing drive in the same manner as in Example5, a high contrast display was performed similarly as in Example 5.

EXAMPLE 7

A liquid crystal device was prepared and evaluated in the same manner asin Example 5 except that another ferroelectric liquid crystal("ZLI-4273" (trade name), available from Merck & Co., Inc. ) was usedinstead of the phenyl-pyrimidine-based mixture liquid crystal. Thecontrast and surface energy values obtained thereby are shown in Table 2below.

As a result of the multiplexing drive in the same manner as in Example5, a high contrast display was performed similarly as in Example 5.

COMPARATIVE EXAMPLE 4

A liquid crystal device was prepared and evaluated in the same manner asin Example 5 except that alignment films were prepared from anotherpolyimide precursor ("SE-100" (trade name), available from Nissan KagakuK.K.). The contrast and surface energy values obtained thereby are shownin Table 2 below.

As a result of the multiplexing drive in the same manner as in Example5, the resultant contrast was lower than in Example 5.

COMPARATIVE EXAMPLE 5

A liquid crystal device was prepared and evaluated in the same manner asin Example 5 except that alignment films were prepared from anotherpolyimide precursor ("LQ-5200" (trade name), available from HitachiKasei K.K.). The contrast and surface energy values obtained thereby areshown in Table 2 below.

As a result of the multiplexing drive in the same manner as in Example5, the resultant contrast was lower than in Example 5.

EXAMPLE 8

A liquid crystal device was prepared and evaluated in the same manner asin Example 5 except that the two substrates were superposed with eachother so that the rubbing direction of the upper substrate deviated by 6degrees clockwise from that of the lower substrate as show in FIG. 14.The contrast and surface energy values obtained thereby are shown inTable 2 below.

As a result of the multiplexing drive in the same manner as in Example5, the resultant contrast was slightly lower than in Example 5.

                  TABLE 2                                                         ______________________________________                                                      Difference due to rubbing                                                       in surface   in dispersion                                            Contrast                                                                              energy (dyne/cm)                                                                           term (dyne/cm)                                   ______________________________________                                        Ex. 6     28:1      14.9         15.6                                         Ex. 7     32:1      11.6         11.2                                         Comp. Ex. 4                                                                              5:1      1.4          0.7                                          Comp. Ex. 5                                                                              4:1      1.1          0                                            Ex. 8     22:1      11.6         11.2                                         ______________________________________                                    

As is understood from the above Examples 5-8 and Comparative Examples4-5, remarkable improvements in contrast, particularly duringmultiplexing drive, were realized by using an alignment film showing asurface energy difference ΔE due to rubbing of at least 9 dyne/cm,preferably 9-20 dyne/cm, and a difference Δδ in dispersion term ofsurface energy due to rubbing of at least 6 dyne/cm, preferably 6-20dyne/cm.

What is claimed is:
 1. An electrode plate for a liquid crystal device,comprising: a substrate, and an alignment film formed on the substrate;wherein said alignment film comprises a fluorine-containing polymerhaving a surface energy of at most 35 dyne/cm before rubbing and hasbeen rubbed to have a refractive index anisotropy of at least 0.02. 2.An electrode plate according to claim 1, wherein said alignment film hasa surface energy of at most 32 dyne/cm.
 3. An electrode plate accordingto claim 1, wherein said alignment film has been rubbed to have arefractive index anisotropy in the range of 0.025-0.075.
 4. A liquidcrystal device, comprising a pair of substrates, at least one of whichhas an alignment film thereon, and a chiral smectic liquid crystaldisposed between the substrates; wherein said alignment film comprises afluorine-containing polymer having a surface energy of at most 35dyne/cm before rubbing and has been rubbed to have a refractive indexanisotropy of at least 0.02.
 5. A device according to claim 4, whereinsaid alignment film has a surface energy of at most 32 dyne/cm.
 6. Adevice according to claim 4, wherein said alignment film has been rubbedto have a refractive index anisotropy in the range of 0.025-0.075.
 7. Adevice according to claim 4, wherein said alignment film subjected torubbing is provided to both substrates.
 8. A device according to claim7, wherein the alignment films provided to both substrates have beenrubbed in directions which are parallel to each other and identical toeach other.
 9. A device according to claim 7, wherein the alignmentfilms provided to both substrates have been rubbed in directions whichare parallel to each other and opposite to each other.
 10. A deviceaccording to claim 7, wherein the alignment films provided to bothsubstrates have been rubbed in directions which are almost identical toeach other but cross each other an intersection angle of 2-20 degrees.11. A device according to claim 4, wherein the alignment film is formedof a polyimide resin.
 12. A device according to claim 4, wherein thealignment film is formed of a polyamide resin.
 13. A liquid crystalapparatus, including:(a) a liquid crystal panel comprising a pair ofsubstrates having thereon scanning electrodes and data electrodes,respectively, so as to form an electrode matrix, at least one of thesubstrates further having thereon an alignment film; and a chiralsmectic liquid crystal disposed between the substrates; and (b) drivemeans for applying a scanning signal to the scanning electrodes andapplying data signals to the data electrodes in synchronism with thescanning signal; wherein said alignment film comprises afluorine-containing polymer having a surface energy of at most 35dyne/cm before rubbing and has been rubbed to have a refractive indexanisotropy of at least 0.02.